1. Field of the Invention
The present invention relates to an oblique incidence interferometer for applying a light obliquely to a target to be measured and measuring the form of the target in accordance with interference of a light reflected from the target with a reference light.
2. Description of the Related Art
A conventional normal incidence interferometer is known as a high-precision measuring technology that uses the wavelength of light as the unit of length. There is a problem in the conventional technology, however, because it is not possible to measure the form of the target having discrete steps larger than half the wavelength or large undulations with a variation in height larger than half the wavelength between adjacent pixels in an image. On the contrary, oblique incidence interferometers have been known as those capable of measuring large roughness (For example, Japanese Patent disclosures Nos, 4-286904, 2000-18912 and 2001-194132). These oblique incidence interferometers apply a light obliquely to the target to obtain a reflected light, thereby reducing the variation in wave front relative to the roughness of the target as intended. By utilizing this feature, the oblique incidence interferometer is able to measure larger rough shapes than the normal incidence interferometer can.
A configuration example of the oblique incidence interferometer of prior art is shown in FIG. 10. This oblique incidence interferometer mainly comprises an illuminator unit 100, and a detector unit 300.
The illuminator unit 100 includes a light source 101, lenses 102, 103, a beam splitter element 104, and a beam synthesizer element 105.
The detector unit 300 includes a lens 301, and an imaging element 302.
The light emitted from the light source 101 travels through the lenses 102, 103 and enters the beam splitter element 104 as a parallel beam, which is split into two beams. One of the split beams is applied obliquely to the surface of a target 200 to be measured. The light reflected from the target 200 is synthesized in the beam synthesizer element 105 with the other of the beams split at the beam splitter element 104. The synthesized beam is lea through the lens 301 and captured as an interference fringe image on the imaging element 302. For beam split and synthesis, a beam splitter and a diffraction grating may be used in general. Another known oblique incidence interferometer comprises an illuminator unit 100′ that includes a triangular prism 106, as shown in FIG. 11, instead of the beam splitter element 104 and the beam synthesizer element 105. This oblique incidence interferometer is configured to apply a laser light through the triangular prism 106 to the target to cause interference between the lights reflected from the prism surface and the target surface.
In the case of the oblique incidence interferometer as described above, the difference in height between adjacent fringes in the interference fringes obtained at the oblique incidence interferometer is represented by λ/(2 cos θ), where θ denotes the incident angle of the laser light from the normal to the target surface, and λ denotes the wavelength of the laser light. Thus, a larger incident angle θ allows measurements to be executed in a wider range than the conventional normal incidence interferometer can.
Analysis of the interference fringe images obtained from FIG. 10 allows the form of the target 200 to be acquired as numeral data. As a method of precisely analyzing interference fringe images, a phase shifting method is used in general. The phase shifting method is a technique of shifting the phase between the beam from the reference surface and the beam from the target, capturing a plurality of interference fringe images, and analyzing the interference fringe images. Shifting the phase of the interference fringe in such the configuration of FIG. 10 requires a process of moving the target relative to the oblique incidence interferometer. Otherwise, it is required to arrange an optical beam-delay element on the optical path of either the reference beam or the measurement beam, or execute a process of shifting the wavelength of the light source.
The phase shift given to the interference fringe by the relative displacement of the target requires a precise displacement of the order of nanometers to be given and causes an error in accordance with the displacement accuracy. Alternatively, the phase shift performed by variations in wavelength requires an expensive light source capable of varying the wavelength precisely in accordance with the measurement accuracy. It also requires a time for shifting the phase of the interference fringe to capture a plurality of interference fringe images. In addition, during acquisition of data, the target must remain stationary relative to the oblique incidence interferometer. Therefore, an occurrence of vibrations in the measurement environment during measurement disables the measurement.
The oblique incidence interferometer of prior art requires a mechanism capable of executing movements with an accuracy of nanometers during the phase shift given to the interference fringe, or an expensive light source capable of varying the wavelength with sufficient accuracy. Such the requirement makes it difficult to produce devices and provides finished products or devices at higher prices. During acquisition of a plurality of interference fringe images required for analysis, targets to be measured are limited to those that remain stationary with the order of nanometers. Therefore, targets and use environments are limited from the nature of the device.